1. Field of the Invention
The present invention relates to a system which automatically inspects the true position and size at point of entry of holes, more specifically, the position and size of cooling holes manufactured in "hot section" turbine blades and vanes of jet engines.
2. Description of the Related Prior Art
Cooling holes are manufactured in airfoils of jet engines to distribute cooling air over the active surface of the airfoil, via a diffuser formation extending laterally and downstream from the discharge end of each of a plurality of the cooling holes.
For a detailed discussion of such holes and methods by which they are manufactured, reference may be had to U.S. Pat. No. 4,197,443. The particular technique of that patent involves electric-discharge machining (EDM) of cooling holes wherein a single electrode is so configured as to form both the cooling hole and its diffusion area. This patent also teaches that a single such electrode may comprise comb-like formations whereby a single EDM stroke may develop and form both the hole and the diffuser for each of an arrayed plurality of holes and associated diffusers in a single blade.
Another process for forming cooling holes in airfoils is laser drilling as taught in U.S. Pat. No. 4,808,785, which discloses a two-step process including laser drilling a hole in an airfoil and subsequently performing an EMD step to form the diffuser shaped part of the hole.
Regardless of how the cooling holes are manufactured in the airfoil, they must be inspected to check their true position as compared to their correct position. The present industry methods for measuring the true position and size at point of entry of cooling holes involves either a surface plate layout method or a comparator method.
In the surface plate layout method, the blade or vane having the cooling holes manufactured therein is mounted in a stage. A scratch line is made on the blade or vane where the correct X coordinate of the hole should be located, and a scratch line is also made on the blade or vane where the correct Y coordinate of the hole should be located. The blade or vane is then inspected to see if the intersection of the scratch lines corresponds to the actual position of the hole. Alternatively, a pin is placed in the cooling hole and the blade or vane is rotated and tilted to an angle so that the pin is parallel to a table upon which the stage holding the blade or vane is seated. The X Y distance of the pin from a specified point on the blade or vane is then measured, and the deviation of the position of the true hole from its correct position can be determined. These types of surface plate layout methods, however, are generally unsatisfactory since they are based upon manual operation of the various elements and, as such, precise inspection cannot be achieved.
In the comparator method, light is reflected off the blade or vane and is projected onto a screen having an overlay showing the correct position of the cooling holes for that blade or vane. Deviation of the true position of the cooling holes from their correct position shows on the overlay. This method, however, is not precise due to the compound reflection angles involved. Furthermore, this method requires precise charts and dedicated tooling.